The present invention relates generally to polyolefin foams and, more particularly, to extruded polyethylene foam sheets having an elastomer-containing film adhered thereto.
Polyolefin foams, particularly polyethylene foams, and methods for manufacturing such foams are well known in the art. See, e.g., U.S. Pat. No. 5,348,984 (Lee), U.S. Pat. No. 5,462,974 (Lee), and U.S. Pat. No. 5,667,728 (Lee), the disclosures of which are incorporated herein by reference thereto. One of the most common polyethylenes used is low density polyethylene (LDPE).
Polyethylene (PE) possesses a number of characteristic physical and chemical properties when used to produce a foamed sheet. Of present interest is the coefficient of friction (COF) of the surface of PE foam sheet, which generally is relatively low. While this property is generally desirable for certain applications, e.g., wave-boards (also known as bodyboards), kick-boards, and other watersport articles, in other applications, the low COF of PE foam is disadvantageous. A particular such application wherein a higher COF would be desired is the use of PE foam for a tool box liner, i.e., a cushion upon which tools may be placed in a tool box to protect both the tools and the tool box, and also to decrease the noise generated when the tool box is moved or otherwise handled. Tool boxes often have individual drawers that are pulled out to provide access to a desired tool. Such movement places a lateral force on the tools at the point at which the tools rest on the surface of the base of the drawer. Absent sufficient frictional force between the tools and the drawer, the tools have a tendency to slide relative to the drawer surface towards the rear of the drawer, thereby accumulating in a disorderly jumble at the rear of the drawer. As can be appreciated, this situation makes it more difficult to locate the intended tool than if the tools were neatly arrayed on the base of the drawer. Similar tool movement can also occur during movement or other handling of the tool box.
Notwithstanding PE foam""s inherently low COF, it is advantageously used as a liner that is disposed at the base of drawers or other flat surfaces within tool boxes, due to its excellent cushioning and sound-dampening capabilities. Such properties provide both noise-reduction and protection to the tools and tool box during movement of the tool box and its component parts, e.g., opening of drawers. The cushioning provided by PE foam tool box liners also protects the tools and tool box as tool users often return their tools to the box during a project by tossing the tools into the tool box. In addition, the closed-cell construction of PE foam is such that dirt, oil, etc. is prevented from penetrating the PE foam liner, and thereby keeps the box and its components from accumulating dirt and oil. Instead, dirty liners are periodically replaced, which is much more convenient than cleaning the box.
However, due to the inherently low COF of PE foam, conventional PE foam leaves much to be desired as a tool box liner, since it allows tools to move around within the tool box as described above instead of holding the tools in place during movement of the box or its component drawers.
Another application in which a higher COF would be desired for PE foam is the use of a non-skid PE foam placed on airplane wings to facilitate servicing of the aircraft. This would protect the wing surface of the aircraft as maintenance personnel walk thereon while minimizing the risk to the maintenance workers of slipping and falling from the wing.
Other applications for PE foam wherein a higher COF would be desirable include the use of PE foam for the packaging of articles to protect them during shipment. For many articles, e.g., interior and exterior automotive parts, a higher COF would help to keep the foam properly in place in relation to the packaged article by increasing the cling or grip between the foam and the article.
Accordingly, a need exists in the art for a PE foam that provides the same cushioning and sound-dampening performance as conventional PE foam, but which has a higher COF in order to allow objects disposed on the foam to remain in place during movement of the foam.
That need is met by the present invention, which provides a composite structure comprising:
a. a foam sheet comprising polyolefin; and
b. a film having an upper surface and a lower surface in adherence with a surface of the foam sheet, the film comprising a blend of polyethylene homopolymer or copolymer and a thermoplastic elastomer, the thermoplastic elastomer comprising a copolymer or terpolymer comprising a styrenic component and a rubbery component, the rubbery component having at least one carbon-carbon double bond and comprising at least about 70 wt. % of the thermoplastic elastomer,
whereby, the film in adherence with the foam sheet results in a coefficient of friction ranging from about 0.5 to about 2.0 as measured at the upper surface of the film.
In accordance with another aspect of the invention, a method for making a composite structure comprises:
a. providing a foam sheet comprising polyolefin; and
b. adhering a film having an upper surface and a lower surface to a surface of the foam sheet, the lower surface of the film being in adherence with the foam sheet, the film comprising a blend of polyethylene homopolymer or copolymer and a thermoplastic elastomer, the thermoplastic elastomer comprising a copolymer or terpolymer comprising a styrenic component and a rubbery component, the rubbery component having at least one carbon-carbon double bond and comprising at least about 70 wt. % of the thermoplastic elastomer,
whereby, the film in adherence with the foam sheet results in a coefficient of friction ranging from about 0.5 to about 2.0 as measured at the upper surface of the film.
The COF range of 0.5 to 2 provided by the composite structure in accordance with the present invention is an increase over that of polyethylene foam alone, and has been found sufficient to maintain tools in place in tool boxes when used as a liner therefor, reduce or eliminate slipping when used as a non-skid foam for, e.g., aircraft maintenance, and keep the composite structure in place on a packaged article when used as a protective packaging wrap. At the same time, the excellent cushioning characteristics of PE foam are retained, so that the tools and tool box, aircraft wing surface, and package articles are protected.